feat(go): using the 'mod-name' flag results in a more-intuitive Go package structure

[asteurer]

This change affects the following scenarios:

• The mod-name flag is used (user wants to create a library), which results in the following
  • The go.mod file is omitted
  • The wit_exports.go file is moved into it's own package
  • To make use of the wit_exports, the user will need to include an import like this:

  import  _ "{mod_name}/wit_exports"

• The mod-name flag is not used (user wants to create a standalone binary), which results in the following
  • A go.mod file is created
  • The wit_exports.go file is the "main" package, and the requisite main function is included.

Also, we no longer need the pkg-name flag!

@dicej